To get bright ones, you'd simply have to selectively breed them. You'd have to cross the brightest golds with each other and then cross the brightest of their offspring and so on. Gradualy, the color would become more and more bright (up to a point). Remember how I described the emergence of platinums in that other thread (the lightest golds were bred to the lightest for many generations...)?
Mutations are not something you can create through selective breeding or inbreeding. They are completely random 'mistakes' made when the fish' DNA is copying itself. All living things harbour mutations (in fact, if mutations did not occur, neither would evolution and we humans wouldn't be here!). Actualy, all fish have mutations but not all are things you can see (If I remember the figure correctly, every human being has, on average, about 100 mutations in their DNA - so you have about 100, I have about 100 etc - but we can't see them because they don't affect our appearance or are 'neutral' in terms of our biology. The same is true of fish.).
Think of it like this: imagine DNA is a rule book that explains exactly how to build a gourami. Now imagine this rule book is copied by one person and the copy given to another and that person copies it as well and passes their copy on and so on... Every time the book is copied by someone, there will be little mistakes in the words. Some of these mistakes may be realy small. Maybe a full stop is missing or a comma is where it shouldn't be. Or maybe they are major changes - like the word 'head' has been changed to 'read'. Some of these 'mistakes' in copying will lead to interesting changes. others, however, will be harmful.
In this sense, the 'interesting' changes may be a new color popping up (like red dwarf gouramies or gold three-spots). The harmful might be a bent spine or a faulty digestive system.
The thing about mutations is that, if they are interesting or useful, you can take advantage of them. For example, think of the 'balloon' kissing gourami. The 'balloon' body shape is, realy, just a deformity caused by a mutation that popped up sometime ago. When people saw this, they liked it and decided to try to breed more. By line breeding the original balloon fish for a few generations, you can produce a strain that always produces 'balloon' kissing gouramies - and now they are quite common. The same would be true of many of the interesting colors we see in dwarf gouramies today - solid red or blue do not occur naturaly in teh wild - but when they popped up in domestic stock, the mutation was retained within the population by line breeding for a few generations.
BTW, by line breeding I mean the original blue/red/balloon fish was bred with another of its species that didn't necessarily display the blue/red/balloon trait. Then some of the fry were bred back to the original blue/red/balloon fish. Normaly, at least some of these second generation fish will display the new characteristic so from then on you can breed fish with similar traits (as usual) to retain the mutation within the gene pool.
You're right that I've been breeding gouramies for a while and I still do it now but, recently, it's been a little more difficult (hopefuly by next year I'll get started again

). I, as you may already know, am especialy 'obsessed' with pearl gouramies and they've been my favourite fish for years and years. I also realy got into fishkeeping when I discovered gouramies because they aren't all too difficult to breed but still challenging and interesting enough behaviour-wise to be rewarding. I haven't actualy seen that many 'good' mutations in the time that I've been breeding gouramies... The vast majority of mutations are hidden ones like the 'neutral' ones I mentioned before that you never actualy know are there and the rest are more often problematic than useful or interesting.
I've had lots of fry born with spinal deformities (as you inevitably do when breeding fish) and a few with swimming difficulties, fin deformities and the like. Some of these may have been considered attractive by some but I wasn't interested in producing more of them at the time. I've had gouramies born with three ventral fins for example. Or even two or three per side (this is actualy quite common in bettas as well).
I also once got a fish with no eyes but, obviously, this wouldn't be something to encourage the breeding of (I should add that she actualy survived for a long time because gouramies have evolved to use their feelers in murky waters and I kept her alone - she died after 3 years by jumping out of the tank. If this fish had been born more recently, I would have euthanised. I don't keep deformed fry alive since I don't have the room to keep them all and I can't expect them to survive in an LFS).
In terms of color mutations, I once saw an opaline with all his color concentrated around his caudal peduncle in a kind of 'half-black' pattern (like you get in guppies). I had not bred him myself but he had resulted from a cross between two of my own fry that a friend had taken on. My friend didn't want to breed him at the time and offered to give him to me. I did breed him once and got some lovely opaline fry but none of them had the same pattern. Realy, what I should have done next was to breed some of his offspring back to him but, unfortunately, I didn't realise this at the time and missed the opportunity and he moved back with my friend. I have no idea where he is now but, actualy, he should still be alive since it's been about 4 years since then.